RIYADH (Reuters) - The United States warned its citizens in Saudi Arabia on Wednesday that militants were planning fresh attacks in the world's biggest oil exporter and security forces said they had found a militant bomb factory.

"The American embassy in Riyadh advises all American citizens living in Saudi Arabia that it has received indications of operational planning for a terrorist attack or attacks in the kingdom," the U.S. embassy in Riyadh said in a statement.

"The embassy has no specific information concerning timing, target or method of any possible attack(s)."

Hours after the embassy published its warning, Saudi Arabia's Interior Ministry said security forces uncovered a hideout 60 km (40 miles) southeast of Riyadh where militants had stored more than two tonnes of bomb-making equipment.

The cache included fertilizer, ammonium nitrate, aluminum powder, potassium nitrate and other chemicals used by suspected al Qaeda militants to make bombs deployed in attacks on Western housing compounds and security forces buildings.

Saudi Arabia has been battling a two-year wave of violence by supporters of Saudi-born al Qaeda leader Osama bin Laden trying to expel Westerners from the country and destabilize the pro-Western royal family.

"CREDIBLE THREATS"

The U.S. embassy statement, which briefly pushed oil prices up 60 cents a barrel, advised Americans in Saudi Arabia to keep a low profile.

"It isn't a result of any specific new threat, except that there are ongoing, credible threats," said Angela Aggelar, spokeswoman for the Consular Affairs division of the State Department in Washington.

"It's just one of our frequent warnings to Americans in the area, to remind them that the security situation there is always pretty grave."

Suicide bombers hit several compounds in 2003 where foreigners are housed and gunmen waged a series of attacks against Westerners last year, including a daylight raid on the U.S. consulate in the Red Sea city of Jeddah.

Saudi security forces have also been targets, although the last high profile strike was a bombing in December outside the Interior Ministry in Riyadh.

Authorities have killed or arrested all but three men on a list of 26 most wanted suspects published in 2003. Last month they issued a list of 36 more wanted men they are still hunting.

One of those suspects, Mohammad Ibrahim al-Hayyari, was killed in a fierce gunbattle in Riyadh two weeks ago.

Militants have killed 91 foreign nationals and Saudi civilians in the last two years and caused more than 1 billion riyals ($270 million) worth of damage, Saudi officials say.

Forty one security force members and 112 militants have also been killed in clashes.